Method and device for controlling photovoltaic inverter, and feed water device

ABSTRACT

A photovoltaic inverter control method includes steps of monitoring a variation in output voltage of a solar battery by a power and voltage monitoring circuit ( 51 ) and, when the variation occurs, accelerating or decelerating an electric motor ( 3 ) to maximize the output voltage of the solar battery ( 1 ), whereby the electric motor for driving, for example, a pump and a fan by the solar battery as a power source can be driven by a photovoltaic inverter always at the maximum power point of the solar battery.

TECHNICAL FIELD

The present invention relates to a method of controlling a photovoltaicinverter for converting and controlling DC power supplied by solar cellsinto AC power (hereinafter referred to as “photovoltaic inverter”) andfor variable-speed controlling an AC motor by making the operating pointof the solar cells to follow the maximum power point to drive the motor,and to a water feeding device for feeding water by, for example, drivinga pump and a fan with a motor variable speed-controlled by means of themethod and device for controlling a photovoltaic inverter forvariable-speed controlling a motor for driving a pump, a fan and thelike.

BACKGROUND ART

The solar cell has a voltage (V)−power (P) characteristics wherein powertends to increase as the amount of insolation increases as shown in FIG.7 when the incident amount of insolation is taken as a parameter,provided that in FIG. 7 Pa, Pb, and Pc represent the maximum powerpoints in the respective amount of insolation.

As a controlling method for extracting efficiently the maximum powerthrough a photovoltaic inverter from the solar cell having such acharacteristic, the so-called maximum power point tracking controlmethod (MPPT control method) is used, wherein the photovoltaic inverteris controlled by generating minute changes in the load of this solarcell and some other means to monitor the ratio of the respectivevariation dP and dV (Δ, Δ=dP/dV) of the output power (P) and voltage (V)of the solar cell, and calculating the point where its polarity changesfrom positive to negative (or from negative to positive) (the maximumpower point) so that the operating point of the solar cell may followthis point.

FIG. 8 is a circuit diagram showing an example of a conventional controldevice of a photovoltaic inverter based on the MPPT control method forcontrolling the output of solar cells. In this figure, 1 represents asolar cell, 2 represents a photovoltaic inverter for controlling theoutput of this solar cell, 3 represents an AC motor driven by thephotovoltaic inverter 2, and 4 represents a mechanical load consistingof, for example, a pump, a fan or the like mechanically connected withthe driving shaft of the motor 3 and rotatively driven by the same. Thedescription below relates to the case of using a pump for the mechanicalload 4.

This photovoltaic inverter 2 includes a voltage detector 21 fordetecting the output voltage (V) of the solar cell 1, a diode 22 forblocking the countercurrent flowing from the inverter main circuit 25described below, a current detector 23 for detecting the current (I)supplied from the solar cells 1, a capacitor 24 for smoothing DCvoltage, an inverter main circuit 25 constituted, for example, by makinga three-phase bridge connection of switching circuits constituted byconnecting back to back a transistor and a diode, and a controllingdevice 26 for controlling to desired values of the voltage and frequencyof the AC output of the inverter main circuit 25 based on the outputvoltage V and the output current I from the solar cell 1 detected by thevoltage detector 21 and the current detector 23.

The control device 26 includes a maximum power point tracking (MPPT)monitoring circuit 31, a rotational speed instructing device 32, anadjustable speed controller 33, a function generator 34, and an invertercontrolling circuit 35. The operation of this control device 26 will bedescribed below with reference to FIG. 9.

In the first place, a start-up frequency instruction value f_(s) isoutputted from the adjustable speed controller 33 based on a start-upinstruction to the solar cell 1 having a voltage−power (V−P)characteristic similar to the one shown in FIG. 9 depending on a givenamount of insolation. Then, the inverter control circuit 35 performspulse width modulation (PWM) operations based on voltage instructionvalues Vs corresponding to this start-up frequency instruction valuef_(s) and those corresponding to the frequency instruction value f_(s)given by a function generator 34 that converts frequency instructionsinto voltage instructions by, for example, a function that converts thevoltage/frequency ratio into a constant value. The control signals basedon the result of such operations allow the ON/OFF control of therespective transistor constituting the inverter main circuit 25. Then,the inverter main circuit 25 generates AC power of a frequency and avoltage corresponding to the start-up frequency instruction value andthe voltage instruction value and accordingly the motor 3 and the pump 4start.

At this time, based on the values detected (V, I) respectively by thevoltage detector 21 and the current detector 23 at previously fixedintervals, the MPPT monitoring circuit 31 calculates the variation (dP)of power (P, P=V×I) and the variation (dV) of voltage (V) and monitorsthe ratioΔ of the power variation to the voltage variation (Δ=dP/dV).However, when the motor 3 and the pump 4 starts as described above, thevoltage of the solar cells 1 drops from a voltage V₁ in the unloadedcondition to a voltage in the loaded condition, and the Δ becomesnegative (Δ<0). This is outputted in the rotational speed instructingdevice 32.

And the rotational speed instructing device 32 outputs a speedinstruction value Δn (constant value) representing a speed increase or aspeed reduction corresponding to the polarity of Δ mentioned above. Inother words, when the Δ is negative (see FIG. 9), a speed increaseinstruction value in the form of a positive +Δn is outputted, and whenthe Δ is positive (see FIG. 9), a speed reduction instruction value inthe form of a negative −Δn is outputted. Therefore, when the Δ is almostnil, the outputted Δn will also be almost nil.

And the adjustable speed controller 33 integrates Δn inputted by therotational speed instructing device 32, and the calculation result isadded to the start-up frequency instruction value f_(s) to be outputtedas a frequency instruction value. When the integrating time is adjustedby the polarity of Δn, the motor current is previously set at a givenvalue so that the instructed speed may be attained as soon as possiblewithout causing the motor current to turn into an overcurrent.

In other words, the voltage amplitude and frequency of the AC poweroutputted from the inverter main circuit 25 rises up to the voltage V₂which will be the maximum power point of the solar cells 1 shown in FIG.9 in response to the output of the MPPT monitoring circuit 31constituting a controlling device 26 after the motor 3 and the pump 4started running by the AC power of the startup voltage and frequencyfrom the inverter main circuit 25, and in response thereto therotational speed of the motor 3 and the pump 4 increases.

When, for some reason, the voltage of the solar cells 1 dropped belowthe V₂ mentioned above, in other words, when it is in a state of havingpassed the maximum power point, both the power P and the voltage V dropand therefore the ratio Δ of variation of the power and voltageoutputted from the MPPT monitoring circuit 31 becomes positive (Δ>0) anda speed reduction instruction is given to the inverter. As a result, themotor 3 reduces its speed and consequently its power consumption, andthe operating point shifts to the side of the maximum power point of thesolar cell. Such a control of the photovoltaic inverter enables to makethe loaded condition of the motor follow the maximum power point of thesolar cell.

According to the conventional controlling method of photovoltaicinverter described above, due to the fact that the integrating time of +and − polarity in the adjustable speed controller 33 is set at aconstant value with reference to a Δn of a constant value outputted bythe rotational speed instructing device 32 in order to perform the MPPTcontrol, the acceleration time (time required to accelerate unitrevolutions) and the deceleration time (time required to decelerate unitrevolutions) of the motor 3 in the vicinity of the maximum power pointare relatively short, they tend to roam around the maximum power pointand caused the control operation to fluctuate. And when the respectiveintegrating time of the + and − polarities is set at a longer value inorder to reduce this fluctuation, a new problem arises in that thesettling time for the MPPT control and the response time accompanying arapid change in the amount of insolation grow longer.

The object of the present invention is to provide a new method ofcontrolling photovoltaic inverters that solves the above-mentionedproblems.

DISCLOSURE OF INVENTION

In order to achieve the object mentioned above, the invention relates toa method of controlling a photovoltaic inverter by variable speedcontrolling a motor powered by a solar cell, wherein the starting powersupplied from the solar cell through the photovoltaic inverter is usedto start the motor while the power and voltage outputted by the solarcell is respectively monitored, and during the process of acceleratingthe rotational speed of the motor for a given acceleration time, thevoltage value of the solar cell when the power of the solar cell hasreached the maximum power point is stored as a reference voltageV_(BASE), and the acceleration is maintained until the voltage of thesolar cell reaches a level (V_(BASE)−V_(OVER)) lower than the referencevoltage V_(BASE) by a previously set voltage range V_(OVER), and afterthe voltage of the solar cell has reached this voltage(V_(BASE)−V_(OVER)), the speed of the motor is decelerated for a givendeceleration time, and after this deceleration operation is over, whenthe voltage of the solar cell is within a given allowable voltageregulation range Hb with reference to the reference voltage V_(BASE),the photovoltaic inverter is controlled in such a way that the motor maycontinue operating at the current rotational speed.

Further, when the voltage of the solar cells has risen beyond the givenallowable voltage regulation range Hb with reference to the referencevoltage V_(BASE) during the continued operation, the rotational speed ofthe motor is accelerated during the given acceleration time until thevoltage of the solar cells reaches the voltage (V_(BASE)−V_(OVER)) lowerthan the reference voltage V_(BASE) by a previously set voltage rangeV_(OVER), and in the process of this acceleration, the value of voltageof the solar cells at the time when the power of the solar cell hasreached the maximum power point is stored as the new reference voltageV_(BASE) 2 and after the voltage of the solar cells has reached thevoltage V_(BASE)−V_(OVER), the photovoltaic inverter is controlled insuch a way that the rotational speed of the motor is decelerated for agiven deceleration time until the voltage of the solar cells shifts fromthis voltage to the new reference voltage V_(BASE) 2.

In addition, when the voltage of the solar cells has fallen beyond thegiven allowable voltage regulation range Hb with reference to thereference voltage V_(BASE) during the continued operation, therotational speed of the motor is decelerated for the given decelerationtime until the voltage of the solar cells reaches the reference voltageV_(BASE), the rotational speed of the motor is accelerated for the givenacceleration time until the voltage of the solar cells reaches a level(V_(BASE)−V_(OVER)) lower than the reference voltage V_(BASE) by apreviously set voltage range V_(OVER), and in the process of thisacceleration, the voltage value of the solar cell at the time when thepower of the solar cell has reached the maximum power point is stored asthe new reference voltage V_(BASE) 3, and after the voltage of the solarcells has reached the voltage V_(BASE)−V_(OVER), the photovoltaicinverter is controlled in such a way that the rotational speed of themotor is decelerated by a given deceleration time until the voltage ofthe solar cells shifts from this voltage to the new reference voltageV_(BASE) 3.

Further still, when the voltage of the solar cells has dropped beyondthe minimum voltage value V_(LOW) tolerated for the solar cells duringthe continuous operation described above, in the first place therotational speed of the motor is rapidly decelerated by a fasterdeceleration time than the given deceleration time mentioned above untilthe voltage of the solar cells reaches the minimum voltage V_(LOW), andthen rotational speed of the motor is decelerated by the givendeceleration time until the voltage of the solar cells reaches thereference voltage V_(BASE), and after the voltage of the solar cells hasreached the reference voltage V_(BASE), the rotational speed of themotor is accelerated for the given acceleration time until the voltageof the solar cells reaches the voltage (V_(BASE)−V_(OVER)) lower thanthe reference voltage V_(BASE) by a previously set voltage rangeV_(OVER), and during this acceleration process the voltage value of thesolar cells prevailing at the time when the power of the solar cell hasreached the maximum power point is stored as the new reference voltageV_(BASE) 4, and after the voltage of the solar cells has reached thelevel (V_(BASE)−V_(OVER)), the photovoltaic inverter is controlled insuch a way that the speed of the motor may decelerate for the givendeceleration time until the voltage of the solar cells shifts from thisvoltage to the new reference voltage V_(BASE) 4.

The motor may be one that drives either a pump or a fan.

In addition, the acceleration time and the deceleration time of themotor for driving the pump or the fan are set at values equivalent tothe cube (n/N_(MAX))³ of the ratio of the current rotational speedinstructing value (n) to the maximum value set (N_(MAX)).

The invention also relates to a controlling device of photovoltaicinverters for variable-speed controlling a motor driven by a solar cellas its power source including a power and voltage monitoring means formonitoring respectively the power and voltage of the solar cells, aninstructing value operation means for outputting acceleration ordeceleration instructions based on the outputs of this power and voltagemonitoring means, a gradient coefficient operation means for outputtingacceleration time and deceleration time, and an adjustable speedcontroller for outputting frequency instructions based on the outputs ofthe instructing value operation means and the gradient coefficientoperation means, wherein, in the process of accelerating the motor, thevoltage value of the solar cell at the time when the power of the solarcell has reached the maximum power point is stored as the referencevoltage V_(BASE), and at the same time the motor is accelerated untilthe voltage of the solar cells shifts from the reference voltageV_(BASE) to a level (V_(BASE) −V_(OVER)) lower than the referencevoltage V_(BASE) by a previously set voltage range V_(OVER), and afterthe voltage of the solar cells has reached the voltage(V_(BASE)−V_(OVER)), the speed of the motor is decelerated until thevoltage of the solar cells shifts from this voltage to the referencevoltage V_(BASE), and when, after this deceleration operation, thevoltage of the solar cells is in the given allowable voltage regulationrange Hb with reference to the reference voltage V_(BASE), thephotovoltaic inverter is controlled in such a way that the motor maycontinue running at the current rotational speed.

The invention still further relates to a water feeding device forfeeding water by driving a pump with a motor variable-speed controlledby a photovoltaic inverter operating on a solar cell as its power sourcewherein, the power and voltage of the solar cells are respectivelymonitored, the motor is started and is accelerated for a givenacceleration time during which process the voltage value of the solarcell at the time when the power of the solar cell has reached themaximum power point is stored as the reference voltage V_(BASE), and atthe same time the motor is accelerated until the voltage Vo of the solarcell passes from the reference voltage V_(BASE) to a level(V_(BASE)−V_(OVER)) lower than the reference voltage V_(BASE) by apreviously set voltage range V_(OVER). When the voltage of the solarcells has reached the voltage (V_(BASE)−V_(OVER)), the motor isdecelerated until the voltage of the solar cells shifts from thisvoltage to the reference voltage V_(BASE). When, following thisdeceleration operation, the voltage of the solar cells is in the givenallowable voltage regulation range Hb in reference to the referencevoltage V_(BASE), the motor is kept operating at the current rotationalspeed to drive the pump and to feed water.

According to the present invention, in allowing the photovoltaicinverter to variable-speed control the motor fed with power from solarcells, the monitoring of changes in the terminal voltage of the solarcells enables to grasp changes in the amount of insolation to the solarcell, to shift promptly to the new maximum power points of the solarcells and improve the stability of the controlling operation in thevicinity of the maximum power point.

In addition, for example, in variable-speed controlling a motor fordriving pumps and fans, it is possible to improve the responsiveness tocontrolling operations by noting that the power required by the motor isproportional to the cube of the rotational speed and by correcting theacceleration and deceleration times of the motor based on that value.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a photovoltaic inverter showing a mode ofcarrying out the present invention.

FIG. 2 is a power-voltage characteristic graph of the solar celldescribing the operation of FIG. 1.

FIG. 3 is a characteristic graph describing the operation of the firstembodiment of the present invention.

FIG. 4 is a characteristic graph describing the operation of the secondembodiment of the present invention.

FIG. 5 is a characteristic graph describing the operation of the thirdembodiment of the present invention.

FIG. 6 is a characteristic graph describing the operation of the fourthembodiment of the present invention.

FIG. 7 is a general characteristic graph of the solar cell.

FIG. 8 is a circuit diagram showing a conventional example of thephotovoltaic inverter.

FIG. 9 is a characteristic graph describing the operation of FIG. 8.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present invention shown in the figures will bedescribed below.

FIG. 1 is a circuit diagram of a photovoltaic inverter showing anembodiment of the present invention, and the components having the samefunction as the conventional circuits shown in FIG. 8 are indicated bythe same codes.

In other words, the photovoltaic inverter 5 shown in FIG. 1 includes acontrolling device 50 in place of the controlling device 26 of theconventional photovoltaic inverter 2 shown in FIG. 8.

The controlling device 50 is composed of, in addition to a functiongenerator 34 and an inverter controlling circuit 35 having the samefunctions as the conventional device, a newly added power and voltagemonitoring circuit 51, an instructing value operation circuit 52, anadjustable speed controller 53 and a gradient coefficient operationdevice 54. And as each of the solar cells 1 have individually avoltage−power (V−P) characteristic that varies as shown in FIG. 2depending on the variation in the amount of insolation. On thecontrolling device 50, although details will be described below, foreach solar cell, the range of voltage drop V_(OVER)from the voltagevalue V_(BASE)at the maximum power point (P₁, P₂, P₃ and the like inFIG. 2), the minimum voltage value V_(LOW) tolerated by the solar cell 1in the V−P characteristic curve of the minimum amount of insolation atwhich the photovoltaic inverter 5 can operate (the minimum amount ofinsolation characteristic curve shown in the figure) as well as theallowable voltage regulation range Hb to changes in voltage arepreviously set as acquired by experiments. In this case, the range ofvoltage drop V_(OVER) must be selected to be a value larger than theallowable voltage regulation range Hb to voltage changes. And to enablean optimum control in response to the usage of the device and theenvironment in which it is installed, the values of the voltage rangeV_(OVER), the minimum voltage value V_(LOW) and the allowable voltageregulation range Hb are set variable as required.

FIG. 3 is a characteristic graph for describing the first controllingmethod of the photovoltaic inverter of the present invention shown inFIG. 1.

In other words, when the solar cell 1 is exposed to a given amount ofinsolation, it shows a V−P characteristic as shown in FIG. 3. When thephotovoltaic inverter is operated together with solar cells having sucha characteristic, in response to a start-up instruction given, theadjustable speed controller 53 outputs a start-up frequency instructingvalue f_(s) corresponding thereto. The inverter controlling circuit 35calculates pulse width modulation (PWM) using this start-up frequencyinstructing value f_(s) and an output voltage instructing value V_(s)obtained by a calculation in which a given function is applied to thisfrequency instructing value f_(s) in the function generator 34, and thecontrol signals formed by the result of this calculation are used tocontrol ON, OFF each of the transistors constituting the inverter maincircuit 25 leading to the output of AC power having a voltage andfrequency corresponding to the voltage instructing value V_(s) and thefrequency instructing value f_(s) at the start-up time from the invertermain circuit 25. And the motor 3 and the pump 4 start up and beginsrunning accordingly.

Then, the power and voltage monitoring circuit 51 calculates atintervals previously set power (P, P=V×1) based on the voltage (V) andthe current (I) outputted by the solar cells 1 and detected by thevoltage detector 21, the current detector 23, and monitors this power(P) and voltage (V).

The power and voltage monitoring circuit 51 basically generates a speedincrease instruction when the power increases in response to a drop inthe voltage monitored, and a speed reduction instruction when the powerdecreases in response to a voltage drop.

When the motor 3 starts up and begins rotating in response to a start-upinstruction, the voltage V of the solar cell drops, as mentioned below,from the prior voltage value V_(NL) representing an unloaded conditionand the power increase as the frequency instructing value (f) outputtedby the adjustable speed controller 53 increases, and therefore the powerand voltage monitoring circuit 51 continues outputting a speed increaseinstruction to the instructing value operation circuit 52.

The instructing value operation circuit 52 to which this speed increaseinstruction has been inputted outputs a previously set speed increaseinstruction +Δn (a constant value) to the adjustable speed controller53. The adjustable speed controller 53 integrates this speed increaseinstructing value +Δn, and outputs the value obtained by adding thestart-up frequency instructing value f_(s) to the calculation result asa frequency instructing value (f).

Noting the fact that the power required for variable-speed controllingthe motor 3 and the pump 4 with power supplied from the solar cells 1 asits power source is proportional to the cube of the rotational speed ofthe motor 3, and in view of the fact that the frequency instructingvalue (f) to the motor 3 outputted by the adjustable speed controller 53is deemed to be a value corresponding more or less to the rotationalspeed of the motor 3, the gradient coefficient operation device 54calculates (f/f_(MAX))³ from the frequency instructing value (f) and themaximum value of the frequency instructing value (f_(MAX)), and inresponse to this value, the integrating time at the time when theadjustable speed controller 53 carries out an integral calculation ofthe Δn is changed. For example, when the frequency instructing value fis a half of the maximum value f_(MAX), the integrating time is (½)³=⅛,in other words, ⅛ of the maximum integrating time is given thereto.

By changing thus the integrating time of Δn depending on the frequencyinstructing value f, it will be possible, in a zone where the frequencyinstructing value f is important, in other words, in a zone where therotational speed is slow, to change gently the rotational speed.

After the motor 3 and the pump 4 started up and began running by an ACvoltage of a voltage and a frequency corresponding to the start-upvoltage instructing value V_(s) and the start-up frequency instructingvalue f_(s) outputted by the inverter main circuit 25 at the time ofstart-up, the adjustable speed controller 53 generates frequencyinstructions (f) by integrating the speed increase instructing value +Δngiven by the instructing value operation circuit 52 by the integratingtime given by the gradient coefficient operation device 54, and thefrequency instructing value (f) outputted from this adjustable speedcontroller 53 increases smoothly. Accordingly, the motor 3 acceleratesmore smoothly.

In this process, the power and voltage monitoring circuit 51 calculatesthe power P based on the values V, I detected respectively by thevoltage detector 21 and the current detector 23 by calculating P=V×I,finds out the point where this calculated power value turns out to bethe maximum point (point Pn on the characteristic curve in FIG. 3),reads the voltage value of the solar cells 1 at this point, stores thesame as the reference voltage V_(BASE), and keeps on outputtingaccelerating instructions until the voltage of the solar cells 1 shiftsfrom the reference voltage V_(BASE) to a voltage lower by a previouslyset range of voltage drop V_(OVER). The maximum point of power can bedetected by comparing in each cycle the power detected recently and thepower detected last time and by finding the turning point from increaseto decrease or from decrease to increase.

Then, when the voltage of the solar cells 1 has shifted from thereference voltage V_(BASE) to the voltage V_(BASE)−V_(OVER) lower by apreviously set range of voltage drop V_(OVER), the power and voltagemonitoring circuit 51 stops outputting the speed increase instructionsand outputs speed reduction instructions to the instructing valueoperation circuit 52. As a result, as stated below, as the frequencyinstructing value (f) outputted by the adjustable speed controller 53 isreduced, the motor 3 decelerates and reduces its load. Accordingly, thevoltage of the solar cells 1 rises, and the power and voltage monitoringcircuit 51 keeps on outputting speed reduction instructions to theinstructing value operation circuit 52 during this period.

The instructing value operation circuit 52 to which this speed reductioninstruction has been inputted outputs to the adjustable speed controller53 the previously set speed reduction instructing value −Δn (a constantvalue), and the adjustable speed controller 53 integrates this speedreduction instructing value −Δn, and outputs the value obtained byadding the frequency instructing value (f) to this calculation result asa new frequency instructing value (f).

In other words, the rotational speed of the motor 3 and the pump 4smoothly decelerates from the rotational speed prevailing at the timewhen the voltage of the solar cells 1 reached V_(BASE)−V_(OVER) as thefrequency instructing value (f) outputted by the adjustable speedcontroller 53 is reduced, and when the voltage V of the solar cell 1 hasrisen to the reference voltage V_(BASE), the power and voltagemonitoring circuit 51 stops outputting speed reduction instructionscausing the adjustable speed controller 53 to stop reducing the value ofthe frequency instructions. As a result, the power output of the solarcell 1 shifts to the maximum power point Pn, where the operationcontinues.

Thereafter, even if the amount of insolation changes and minutedisturbances occurs to the pump 4, when the voltage of the solar cells 1monitored by the power and voltage monitoring circuit 51 is limited, asshown in FIG. 3, to changes within the given allowable voltageregulation range Hb (=V_(BASE)±Hb) with reference to the referencevoltage V_(BASE), the motor 3 and the pump 4 continue running based onthe frequency instructing value (f) valid during their operation at thereference voltage V_(BASE). Therefore, they can keep up their stableoperation without any fluctuations of loads in the vicinity of themaximum power point.

The value of voltage drop range V_(OVER) from the reference voltageV_(BASE) in the process of speed increase at the time of start-upexceeds the allowable voltage regulation range Hb in reference to thereference voltage V_(BASE) set as the zone of insensibility to themonitoring voltage in the power and voltage monitoring circuit 51.Therefore, a value greater than this value is chosen for the same.

FIG. 4 is a characteristic graph for describing the embodiment 2 of thepresent invention. This characteristic graph shows that the amount ofinsolation increased during a continuous operation in the vicinity ofthe maximum power point in the first embodiment described with referenceto FIG. 3 and that the voltage and power characteristic of the solarcells 1 has changed from the characteristic curve α to thecharacteristic curve β.

As such an increase in the amount of insolation leads to an increasedoutput voltage of the solar cells 1, the operating point shifts from thepoint P_(A) of the characteristic curve α to the point P_(B) of thecharacteristic curve β in FIG. 4. As a result, the voltage V of thesolar cell 1 monitored by the power and voltage monitoring circuit 51has increased in excess of the allowable voltage regulation range Hbwith reference to the reference voltage V_(BASE) (shown as V_(BASE) 1 inFIG. 4) in FIG. 3 (V>V_(BASE) 1+Hb), and the power and voltagemonitoring circuit 51 outputs a new speed increase instruction. Thisspeed increase instruction continues to be outputted until the voltageof the solar cell 1 drops to the voltage V_(BASE) 1−V_(OVER) asdescribed below.

Therefore, as the frequency instructing value (f) outputted by theadjustable speed controller 53 increases, the photovoltaic inverter 25increases its output voltage and frequency to accelerate smoothly themotor 3 and the pump 4. During this acceleration process, the power andvoltage monitoring circuit 51 calculates power P (P=V×I) based on therespective outputs (V, I) of the voltage detector 21 and the currentdetector 23, finds the maximum value (point Pc on the characteristiccurve β in FIG. 4) of this calculated power value, stores the voltagevalue of the solar cells 1 at this maximum power point as a newreference voltage V_(BASE) 2, and at the same time accelerates the motorspeed until the voltage V of the solar cell 1 shifts from the referencevoltage V_(BASE) 1 to the voltage V_(BASE) 1−V_(OVER) lower by apreviously set range of voltage drop V_(OVER).

Then, when the voltage V of the solar cell 1 has reached V_(BASE)1−V_(OVER), the power and voltage monitoring circuit 51 stops outputtingthe speed increase instructions, outputs a speed reduction instructionto the instructing value operation circuit 52, and keeps on outputtingthe speed reduction instruction to the instructing value operationcircuit 52 until the voltage V rises to the new reference voltageV_(BASE) 2 from V_(BASE) 1−V_(OVER) as the adjustable speed controller53 reduces the frequency instructing value (f) as described below.

In other words, the rotational speed of the motor 3 and the pump 4prevailing at the time when the voltage V of the solar cells 1 isV_(BASE) 1−V_(OVER) decelerates smoothly as the frequency instructingvalue (f) outputted by the adjustable speed controller 53 is reduced,and when the voltage of the solar cells 1 has risen to the V_(BASE) 2,the power and voltage monitoring circuit 51 stops outputting the speedreduction instructions, and then the adjustable speed controller 53 alsostops reducing the frequency instructing values outputted by the same.

Then, when the variation of the voltage V of the solar cells 1 monitoredby the power and voltage monitoring circuit 51 is within the givenallowable voltage regulation range Hb (V=V_(BASE) 2±Hb) in reference tothe reference voltage V_(BASE) 2 as shown in FIG. 4, the motor 3 and thepump 4 keeps on running based on the frequency instructing value (f)when the speed reduction instruction is suspended.

FIG. 5 is a characteristic graph for describing the third embodiment ofthe present invention. This characteristic graph shows that the amountof insolation decreased during a continuous operation in the vicinity ofthe maximum power point in the first embodiment described with referenceto FIG. 3 and that the voltage and power characteristic of the solarcells 1 has changed from the characteristic curve α to thecharacteristic curve γ in FIG. 5.

In other words, as a decrease in the amount of insolation leads to areduced output voltage of the solar cells 1, the operating point shiftsfrom the point P_(A) of the characteristic curve α in FIG. 4 to thepoint P_(D) of the characteristic curve γ. As a result, the voltage V ofthe solar cell 1 monitored by the power and voltage monitoring circuit51 has dropped in excess of the allowable voltage regulation range Hb inreference to the reference voltage V_(BASE) (shown as V_(BASE) 1 in FIG.5) in FIG. 3 (V<V_(BASE) 1−Hb), and the power and voltage monitoringcircuit 51 outputs a new speed reduction instruction. This speedreduction instruction continues to be outputted until the voltage V ofthe solar cells 1 rises to the reference voltage V_(BASE) 1 as describedbelow.

In other words, in response to a speed reduction instruction outputtedby the power and voltage monitoring circuit 51, the frequencyinstructing value (f) outputted by the adjustable speed controller 53 isreduced, and this is followed by a corresponding reduction in thefrequency and voltage outputted by the photovoltaic inverter 25, causingthe motor 3 and the pump 4 to decelerate smoothly. And when the voltageof the solar cells 1 has reached the reference voltage V_(BASE) 1 , thepower and voltage monitoring circuit 51 stops outputting speed reductioninstructions, the adjustable speed controller 53 also stops reducing itsoutput of frequency instructing values (f), and the rotational speed ofthe motor 3 remains fixed at a rotational speed determined by thevoltage V of the solar cells 1 serving as the reference voltage V_(BASE)1.

Then, when the motor 3 and the pump 4 reaches a rotational speeddetermined by the voltage V of the solar cells 1 serving as thereference voltage V_(BASE) 1 , the power and voltage monitoring circuit51 outputs a new speed increase instruction, and as the frequencyinstructing value (f) outputted by the adjustable speed controller 53increases, the output frequency and voltage of the photovoltaic inverter25 increase, and the motor 3 and the pump 4 accelerate smoothly. At thistime, the power and voltage monitoring circuit 51 calculates power P(P=V×I) based on the values V, I detected respectively by the voltagedetector 21 and the current detector 23, and the detected value of thevoltage V of the solar cell 1 at the point (point P_(E) on thecharacteristic curve γ in FIG. 5) where this calculated power value willbe the maximum value) is stored as the new reference voltage V_(BASE) 3, and the motor 3 and the pump 4 will be accelerated until voltage V ofthe solar cells 1 reaches V_(BASE) 1−V_(OVER).

Then, when the voltage V of the solar cell 1 has reached V_(BASE)1−V_(OVER), the power and voltage monitoring circuit 51 stops outputtingthe speed increase instructions, outputs a speed reduction instructionto the instructing value operation circuit 52. As a result, as statedbelow, following a drop in the frequency instructing value (f) outputtedby the adjustable speed controller 53, the voltage V of the solar cells1 rises to a new reference voltage V_(BASE) 3. During this period, thepower and voltage monitoring circuit 51 continues outputting this speedreduction instructions to the instructing value operation circuit 52.

In other words, the rotational speed of the motor 3 and the pump 4prevailing at the time when the voltage V of the solar cell 1 isV_(BASE)−V_(OVER) decelerates smoothly as the frequency instructingvalue (f) outputted by the adjustable speed controller 53 is reduced,and when the voltage V of the solar cell 1 has risen to the referencevoltage V_(BASE) 3, the power and voltage monitoring circuit 51 stopsoutputting the speed reduction instructions, and then the adjustablespeed controller 53 also stops reducing its frequency instructionoutput.

Then, when the voltage V of the solar cells 1 monitored by the power andvoltage monitoring circuit 51 is within the given allowable voltageregulation range Hb (V≦V_(BASE) 3±Hb) with reference to the newreference voltage V_(BASE) 3, the motor 3 and the pump 4 keeps onrunning based on the frequency instructing value (f) when the speedreduction instruction is suspended.

FIG. 6 is a characteristic graph for describing the fourth embodiment ofthe present invention. This characteristic graph shows that the amountof insolation decreased sharply during a continuous operation in thevicinity of the maximum power point in the first embodiment describedwith reference to FIG. 3 and that the power and voltage characteristicof the solar cell 1 has changed from the characteristic curve α to thecharacteristic curve δ in FIG. 6.

In other words, when the amount of insolation decreases rapidly to thebottom, and, as shown in FIG. 6, due to the characteristics of the solarcells 1, the operating point shifts from the point P_(A) of thecharacteristic curve α to the point P_(F) of the characteristic curve δ,the voltage of the solar cell 1 monitored by the power and voltagemonitoring circuit 51 drops below the minimum voltage value V_(LOW)already fixed. As a result, the power and voltage monitoring circuit 41outputs a rapid speed reduction instruction. In response to this rapidspeed reduction instruction, the instructing value operation circuit 52gives to the adjustable speed controller 53 a speed reductioninstructing value −Δn for reducing the frequency instructing value (f)and for increasing rapidly the voltage V of the solar cell 1 to theminimum voltage V_(LOW) (point P_(G) in FIG. 6).

Then, when the operating point shifts to the point P_(G) on thecharacteristic curve δ, the power and voltage monitoring circuit 51outputs a new speed reduction instruction. This speed reductioninstruction continues being outputted until the voltage V of the solarcell 1 reaches the reference voltage V_(BASE) 1 at the maximum powerpoint on the characteristic curve α.

In other words, following a reduction in the frequency instructing value(f) outputted by the adjustable speed controller 53 in response to thespeed reduction instruction, the motor 3 and the pump 4 deceleratesmoothly, and as a result, when the rotation speed is reduced to that atwhich voltage V of the solar cell 1 becomes the reference voltageV_(BASE) 1, the power and voltage monitoring circuit 51 gives a newspeed increase instruction, and the motor 3 and the pump 4 acceleratesmoothly following an increase in the frequency instructing value (f)outputted by the adjustable speed controller 53. At this time, the powerand voltage monitoring circuit 51 calculates power P (P=V×I) based onthe values V, I detected respectively by the voltage detector 21 and thecurrent detector 23, and stores the value detected of the voltage V ofthe solar cell 1 at the point (point P_(H) on the characteristic curve δin FIG. 6) where this calculated value of power will be maximum as a newreference voltage V_(BASE) 4, and accelerates until the voltage V of thesolar cell 1 reaches V_(BASE) 1−V_(OVER).

Then, when the voltage V of the solar cell 1 has reached V_(BASE)1−V_(OVER), the power and voltage monitoring circuit 51 stops outputtingthe speed increase instructions, outputs a speed reduction instructionto the instructing value operation circuit 52. As a result, following adrop in the frequency instructing value (f) outputted by the adjustablespeed controller 53, as described below, the voltage V of the solarcells 1 rises to a new reference voltage V_(BASE) 4. During this period,the power and voltage monitoring circuit 51 continues outputting thisspeed reduction instructions to the instructing value operation circuit52.

In other words, the rotational speed of the motor 3 and the pump 4prevailing at the time when the voltage V of the solar cell 1 isV_(BASE) 1−V_(OVER) decelerates smoothly as the frequency instructingvalue (f) outputted by the adjustable speed controller 53 is reduced,and when the voltage V of the solar cell 1 has risen to a new referencevoltage V_(BASE) 4, the power and voltage monitoring circuit 51 stopsoutputting the speed reduction instructions, and then the adjustablespeed controller 53 also stops reducing its output.

Then, when the voltage V of the solar cell 1 monitored by the power andvoltage monitoring circuit 51 is, as shown in FIG. 5, within a givenallowable voltage regulation range Hb ((V≦V_(BASE) 4±Hb) with referenceto the reference voltage V_(BASE) 4, the motor 3 and the pump 4 keeps onrunning based on the frequency instructing value (f) when the speedreduction instruction is suspended.

Incidentally, in the first through the fourth embodiments describedabove, the case of prolonging the acceleration and deceleration times inzones where the rotational speed of the motor 3 and the pump 4 is fastand of varying gently the rotational speed by using the adjustable speedcontroller 53 and the gradient coefficient operation device 54 isdescribed. However, for uses involving few and gentle variations in theamount of insolation, the acceleration and deceleration times of themotor 3 and the pump 4 can be set at a constant value.

When such a controlling method of photovoltaic inverter is applied tothe adjustable speed control of motors driving water feeding pumps, itis possible to design a water feeding device provided with solar cellsas its power source. Such a water feeding device can operate byfollowing the maximum power point of the solar cells in each moment evenif the amount of insolation towards the solar cells fluctuates, andtherefore it is possible to take the maximum advantage of the output ofthe solar cells even if the quantity of water supplied may changedepending on the sunshine available, and therefore the efficiency ofwater feeding will be very high vis-à-vis the amount of insolation.

INDUSTRIAL APPLICABILITY

According to the present invention, when a motor is to be variable-speedcontrolled by means of a photovoltaic inverter fed with power from solarcells, due to an arrangement that made it possible to monitor anychanges in the terminal voltage of solar cells and, in case of anychange in the amount of insolation towards the solar cells, to controlthe photovoltaic inverter so that it may be operated at the maximumpower point for that amount of insolation, it is possible to eliminateany fluctuations of control in the vicinity of the maximum power pointat each moment of the solar cells, to maintain stability in theiroperation and to improve response time in connection with rapid changesin the amount of insolation.

1. A method of controlling a photovoltaic inverter for variable-speedcontrolling a motor with solar cells as its power source comprising astep of starting up the motor by supplying a start-up power through thephotovoltaic inverter from said solar cells while monitoring the outputpower and voltage of said solar cells, a step of storing the voltagevalue of the solar cells as a reference voltage (V_(BASE)) when thepower of said solar cells reached the maximum power point in the processof accelerating the rotational speed of this motor for a givenacceleration time, a step of continuing accelerating until the voltageof the solar cells shifts from said reference voltage (V_(BASE)) to afirst voltage (V_(BASE)−V_(OVER)) that is lower than said referencevoltage by a previously set voltage (V_(OVER)), a step of, after thevoltage of the solar cells reached the first voltage(V_(BASE)−V_(OVER)), decelerating the speed of said motor for a givendeceleration time until the voltage of the solar cells shifts from thisvoltage to said reference voltage (V_(BASE)), and a step of, after thisdeceleration operation is over, when the voltage of the solar cells iswithin the given allowable voltage regulation range (Hb) with referenceto said reference voltage (V_(BASE)), controlling said photovoltaicinverter in such way that said motor may continue operating at thecurrent speed.
 2. The method of controlling a photovoltaic inverteraccording to claim 1 wherein, during said continuous operation, when thevoltage of the solar cells rises beyond the given allowable voltageregulation range (Hb) with reference to said reference voltage(V_(BASE)), the rotational speed of the motor is accelerated for a givenacceleration time until the voltage of the solar cells shifts from saidreference voltage (V_(BASE)) to the first voltage (V_(BASE)−V_(OVER)),and the voltage value of the solar cells prevailing at the time when thepower of the solar cells reached the maximum power point in thisacceleration process is stored as a new reference voltage (V_(BASE) 2),and after the voltage of the solar cells reached said first voltage(V_(BASE)−V_(OVER)), said photovoltaic inverter is controlled in such away that the rotational speed of said motor may be decelerated by agiven deceleration time until the voltage of the motor shifts from thisvoltage to said new reference voltage (V_(BASE) 2).
 3. The controllingmethod of a photovoltaic inverter according to claim 1 wherein, duringsaid continuous operation, when the voltage of said solar cells dropsbeyond said allowable voltage regulation range (Hb) with reference tosaid reference voltage (V_(BASE)), the rotational speed of said motor isdecelerated by a given deceleration time until said voltage of the solarcells reaches said reference voltage value (V_(BASE)), the rotationalspeed of said motor is accelerated for a given acceleration time untilthe voltage of said solar cells shifts from the reference voltage(V_(BASE)) to the first voltage (V_(BASE)−V_(OVER)), and the voltagevalue of the solar cells prevailing at the time when the power of saidsolar cells reached the maximum power point in this acceleration processis stored as a new reference voltage (V_(BASE) 3), and after the voltageof said solar cells reached said first voltage (V_(BASE) −V_(OVER)),said photovoltaic inverter is controlled in such a way that therotational speed of said motor may be decelerated by a givendeceleration time until the voltage of the motor shifts from thisvoltage to said new reference voltage (V_(BASE) 3).
 4. The controllingmethod of a photovoltaic inverter according to claim 1 wherein, duringsaid continuous operation, when the voltage of said solar cells dropsbeyond a minimum voltage value (V_(LOW)) tolerated by solar cells, therotational speed of said motor is rapidly decelerated by a decelerationtime shorter than the given deceleration time until the voltage of saidsolar cells reaches said minimum voltage value (V_(LOW)), then therotational speed of said motor is decelerated by said given decelerationtime until the voltage of said solar cells reaches said referencevoltage (V_(BASE)), and after the voltage of said solar cells reachedsaid reference voltage (V_(BASE)), the rotational speed of said motor isaccelerated by a given acceleration time until the voltage of said solarcells shifts from this voltage to the first voltage (V_(BASE)−V_(OVER)),and the voltage value of said solar cells prevailing at the time whenthe power of said solar cells reached the maximum power point in thisacceleration process is stored as a new reference voltage (V_(BASE) 4),and after the voltage of said solar cells reached said first voltage(V_(BASE)−V_(OVER)), said photovoltaic inverter is controlled in such away that the speed of said motor may be decelerated by the givendeceleration time until the voltage of the motor shifts from thisvoltage to said new reference voltage (V_(BASE) 4).
 5. The controllingmethod of a photovoltaic inverter according to claim 1, wherein saidmotor is a motor for driving a pump or a fan.
 6. The controlling methodof a photovoltaic inverter according to claim 5 wherein the accelerationtime and the deceleration time of the motor for driving said pump or fanis set at a value equivalent to the cube of the ratio (n/N_(MAX))³ ofthe present rotational speed instructing value (n) to the maximum setvalue (N_(MAX)) of the rotational speed of this motor.
 7. Thecontrolling method of a photovoltaic inverter according to claim 2,wherein said motor is a motor for driving a pump or a fan.
 8. Thecontrolling method of a photovoltaic inverter according to claim 3,wherein said motor is a motor for driving a pump or a fan.
 9. Thecontrolling method of a photovoltaic inverter according to claim 4,wherein said motor is a motor for driving a pump or a fan.
 10. Acontrolling device of a photovoltaic inverter for variable-speedcontrolling a motor fed with power from solar cells comprising power andvoltage monitoring means for monitoring respectively the power andvoltage of said solar cells, an instructing value operation means foroutputting acceleration or deceleration instructions based on this powerand voltage monitoring means, a gradient coefficient operation means foroutputting acceleration time or deceleration time, an adjustable speedcontrolling means for outputting frequency instructions based on theoutputs of the instructing value operation means and the gradientcoefficient operation means, wherein the voltage value of said solarcells prevailing at a time when the power of said solar cells reachedthe maximum point in the process of accelerating said motor is stored asa reference value (V_(BASE)), at the same time the motor is accelerateduntil the voltage of said solar cells shifts from said reference voltage(V_(BASE)) to first voltage (V_(BASE)−V_(OVER)) that is lower than thereference voltage by a previously set voltage (V_(OVER)), and after thevoltage of said solar cells reached said first voltage(V_(BASE)−V_(OVER)) the motor is decelerated until the voltage of saidsolar cells reaches said reference voltage (V_(BASE)), and after thisdeceleration operation, when the voltage of said solar cells is withinthe given allowable voltage regulation range (Hb) with reference to saidreference voltage (V_(BASE)), the photovoltaic inverter is controlled insuch a way that said motor may continue operating at the presentrotational speed.
 11. A water feeding device for feeding water bydriving a pump variable-speed controlled by a photovoltaic inverter fedwith power from solar cells wherein, in the process of starting up saidmotor and accelerating said motor by a given acceleration time whilemonitoring respectively the power and voltage of said motor, the voltagevalue of said solar cells prevailing at a time when the power of saidsolar cells has reached the maximum power point is stored as a referencevoltage (V_(BASE)), and the motor is accelerated until the voltage (Vo)of said solar cells shifts from said reference voltage (V_(BASE)) to afirst voltage (V_(BASE)−V_(OVER)) that is lower than the referencevoltage by a previously set voltage (V_(OVER)), and after the voltage ofsaid solar cells has reached said first voltage (V_(BASE)−V_(OVER)), themotor is decelerated until the voltage of said solar cells shifts fromthis voltage to said reference voltage (V_(BASE)), and after thisdeceleration operation is over, when the voltage of said solar cells iswithin the given allowable voltage regulation range (Hb) with referenceto said reference voltage (V_(BASE)), said motor is kept operating atthe present rotational speed to drive the pump and to feed water.